Critical behavior of superfluidHe4films adsorbed in aerogel glass

Abstract

We present measurements of the superBuid density and heat capacity of thin films of He adsorbed in aerogel glass. We find that the critical behavior of the super8uid density for films with transition temperatures between 50 mK and 1 K is similar to that seen for Alms adsorbed on Vycor. Power-law behavior is observed over at least one order of magnitude in reduced temperature with an exponent ( = 0.63 + 0.03. This exponent is much smaller than the value 0.811 + 0.004 found when the pores were filled with helium. We also find that the exponent is unchanged by the addition of small amounts of He. These results are consistent with the existence of a correlation length that diverges at the superfluid transition. Unlike for full-pore He aerogel, the correlation length is long compared with all structural length scales throughout the superQuid density power-law regime, thus leading to the more bulklike critical behavior. The heat capacity of He films in aerogel shows no singularity like that seen for full-pore He aerogel or for thin films adsorbed in Vycor. We show that this result is consistent with the predictions of hyperuniversality given the long correlation lengths implied by the super8uid density measurements.

Onset of superfluidity inHe4films adsorbed on disordered substrates

“…Although the existence of a non-zero critical coverage n c has been known since the earliest experiments on unsaturated 4 He films, the onset of superfluidity as a function of coverage at T = 0 has received much less experimental attention than the superfluid transition at T = T c . This has followed in part from the difficulty in making thermodynamic measurements on extremely thin 4 He films, but also from a conviction that the nonsuperfluid film of coverage n c forms an essentially inert pseudo-substrate for the overlying superfluid film.…”

“…For a wide variety of substrates, adsorbed films of 4 He thicker than approximately two atomic layers are superfluid at zero temperature. As the temperature is increased, a transition occurs to a non-superfluid phase at a critical temperature T c .…”

“…As the temperature is increased, a transition occurs to a non-superfluid phase at a critical temperature T c . Although the details of this transition depend on the topology of the substrate, [1][2][3][4] the phase diagrams for films adsorbed on substrates such as Mylar, porous glasses, or packed powders are similar. In each case, there is a line of transition temperatures in the density-temperature plane separating non-superfluid from superfluid films.…”

“…Our investigation focused on the onset of superfluidity at low temperatures as the 4 He coverage is increased. Torsional oscillator measurements of the 4 He-aerogel system were used to determine the superfluid density of films with transition temperatures as low as 20 mK.…”

“…We have studied 4 He films adsorbed in two porous glasses, aerogel and Vycor, using high precision torsional oscillator and DC calorimetry techniques. Our investigation focused on the onset of superfluidity at low temperatures as the 4 He coverage is increased.…”